Previous fix (b1773459) tried to fix this, but it seems that
depending on CPU and video width, ffmpeg might need at least 64
byte alignment, even if CPU we're running on is only AVX2 (32 byte
alignment). That is because some other parts of ffmpeg code
statically pick 64 or 32 byte alignment internally, depending on whether
AVX512 support is even compiled in (even if CPU might not have it).

I have checked whether this does not negatively affect a platform where
SIMD alignment is always 16 (Mac), and it does not, everything works as
expected.

Pull Request: https://projects.blender.org/blender/blender/pulls/128107